Currently, electrical wiring is installed through open areas in building structures by first drilling holes through wooden frame elements such as studs (upright wall supports), joists (long horizontal supports that floor boards or ceiling laths are fastened to), stringers (long horizontal supports) and rafters (sloping roof beams). Wiring is then passed through the drilled holes. This process is time consuming, requires the use of a drill, and weakens the support members that must be drilled-through.
Another common method for supporting wiring in a horizontal orientation within a building without having to drill holes in support members is to simply lay the wiring across the stringers and/or joists that form the unimproved "floor" of an attic. However, attic wiring is often subjected to extreme high and/or low temperatures and may be subjected to substantial temperature swings according to seasonal temperature variations experienced in most parts of the world. Temperature extremes and/or substantial temperature variations of this kind can shorten the life of materials used to insulate electrical wiring. Temperature variations also complicate the wiring design of a building by changing the ampacity ratings of wire conductors. In addition, it is hazardous for workers to attempt to service wiring that is supported in this way. Servicing attic wiring is hazardous because, to reach the wires, workers must sometimes attempt to walk across and stand on unimproved attic flooring, i.e., stringers or floor joists with no actual floorboards covering them.
To facilitate the suspension of electrical wiring in buildings in general and the horizontal suspension of electrical wiring in particular, various types of support apparatus in the form of duct, channel, hanger or conduit systems are employed. Such suspension means allow installers to route and support wires beneath frame structures such as ceiling joists and stringers that are disposed above inhabited, temperature-controlled areas of a building--areas that service personnel can more readily gain access to.
One example of such a system is disclosed in U.S. Pat. No. 2,917,083 issued Dec. 15, 1959 to Duvall et al. The Duvall et al. system includes a fully enclosed duct used in commercial construction to support electrical wires. A generally rectangular-shaped coupler connects lengths of the duct end-to-end. Duvall et al. also discloses a duct hanger having a fastening flange with holes through which fasteners such as nails may be driven into an external support structure such as a wooden joist or stringer. The system also includes an end-cap section for closing-off the open end of a duct. Each of the above components of the Duvall et al. system is made of sheet metal--a material that can be expensive and difficult to form and that can present problems in electrical wiring applications due to its high conductivity and characteristically sharp edges. In addition the hangers are two-piece L-shaped units that are relatively difficult to install, as they must be fastened together and to the duct using separate fastening hardware. The couplers are also relatively difficult to install and are also difficult to construct. The couplers are difficult to construct because they must be cut or stamped from sheet metal in two pieces then joined together at a hinged connection. The couplers are difficult to install because they must be fastened within and between two lengths of duct by separate fastening hardware. Still further, these hangers are limited in that they must be positioned at joints between duct lengths to engage studs also used to connect the couplers to the duct ends. In addition, because a completely enclosed duct supports the wiring, the system cannot be used to route certain types of nonmetallic sheathed cable without violating Article 336 of the National Fire Protection Association (NFPA) National Electrical Code. Specifically, types NM and NMC nonmetallic sheathed cable may not be supported within enclosed ducting. Unfortunately, NM-type cable is very common in residential home construction.
Another example of such a system is disclosed in U.S. Pat. No. 1,992,574 issued Feb. 26, 1935 to Jenkins. Similar to the Duvall et al. system, the Jenkins system includes fully enclosed duct sections with box-shaped cross-sections used in commercial construction to support electrical wires. Couplers in the form of end flanges integrally formed at the ends of each length of the duct are configured to join duct sections. Jenkins also discloses a duct hanger with a generally rectangular frame section and an integral fastening flange with holes through which fasteners such as nails may be driven into an external support structure. The system also includes an end-cap section for closing-off the open end of a duct. As with the Duvall et al. system, each of the above components is made of sheet metal and therefore has all the same attendant limitations. The Jenkins hangers are single piece units but must be fastened to end-flange portions of the ducts using separate fastening hardware. The coupler flanges make the duct forming process more difficult as they extend integrally outward from the ends of the ducts. To couple separate lengths of duct the end flanges are fastened together using separate fastening hardware. In addition, the hangers must be assembled to the duct at the same time as the duct lengths are being joined together. This is because the hangers must be positioned between the end flanges and are held in place by the same fastening hardware that joins the end flanges together. As with the Duvall et al. system, the fully enclosed duct of the Jenkins system makes it incompatible with certain non-metallic sheathed cable.
Because of the inherent problems associated with using metal components to support electrical wiring, some current systems include wire suspension components made of plastic. One such system is disclosed in U.S. Pat. No. 4,857,670, issued Aug. 15, 1989 to Frank et al. Frank et al. discloses fully enclosed PVC duct sections for supporting electrical wiring. Couplers in the form of tabs join the duct sections together end-to-end. Opposite ends of each tab are slidably received into grooves formed within each end of each duct section. The tabs and grooves are configured to join duct sections end-to-end in either linear fashion or at right angles. The duct sections include an integrally hinged cover that snaps into place over a U-shaped trough to form the fully enclosed duct with a generally rectangular cross-section. The grooves, the hinged cover and the snap-fit connection of each duct section are relatively complex and difficult to extrude or mold. In addition, the Frank et al. system has the same incompatibility problem with certain types of non-metallic sheathed cable as do the Duvall et al. and Jenkins systems.
What is needed is an electrical wiring support assembly that is inexpensive to manufacture and easy to assemble and install without weakening building structures. What is also needed is such an assembly that can be used with types NM and NMC non-metallic sheathed cable without violating the NFPA National Electrical Code. Also needed is such an assembly that does not create or increase the risk of short-circuiting the electrical wires that it supports.